There are two specific aims of this project: (1) to increase the number of genes mapped on human chromosomes by extensive family studies utilizing rare chromosome variants and common chromosome polymorphisms. Loci to be linked to the identifiable homologues include the HL-A system in our laboratory and 25 red cell, serum, and saliva markers in the laboratory of Dr. Lovrien; (2) to determine the parental origin of the extra chromosome in free trisomy 21 Down's Syndrome patients; further, to determine if the error is a first meiotic error or after meiosis I. New chromosome banding techniques have permitted intra-pair chromosome identification; these differences between homologues appear to be heritable. By finding which parent contributed two chromosomes 21, we will know the parental origin; two different chromosomes 21 from a parent should indicate a first meiotic error, while two identical, a second meiotic or post-zygotic miotic error.